Within the Milky Way, the Earth resides on the inner edge of a spiral arm.

在銀河系中，地球位於旋臂的內側邊緣。

But this isn't our permanent address.

但這不是我們的永久地址。

When most dinosaurs went extinct, our solar system was in a different arm than it is today.

當大多數恐龍滅絕時，我們的太陽系處於與今天不同的臂彎。

The Milky Way never stops changing.

銀河從未停止變化。

Even its structure continues to evolve.

甚至其結構也在不斷演變。

To see why, let's start at our galaxy's very beginning.

要想知道原因，讓我們從銀河系的最初開始說起。

Thirteen billion years ago, propelled by supernovae and the general chaos of the early universe, the gas and dust particles that eventually became our Milky Way were whizzing around in every direction.

130 億年前，在超新星和早期宇宙大混沌的推動下，最終成為我們銀河系的氣體和塵埃粒子在各個方向呼嘯而過。

Each particle had a certain angular momentum— the magnitude and direction of its rotation around the emerging galaxy's center.

每個粒子都有一定的角動量--即圍繞新出現的星系中心旋轉的幅度和方向。

Just as the new galaxy's total mass was the sum of the individual particle's masses, the galaxy also had an angular momentum that was the sum of the particle's angular momenta.

正如新星系的總品質是各個粒子品質的總和一樣，星系的角動量也是粒子角動量的總和。

So, despite the wild motion of its parts, the galaxy as a whole was rotating about an axis.

是以，儘管星系的各個部分都在瘋狂地運動，但星系作為一個整體還是在圍繞著一條軸線旋轉。

Meanwhile, the gas and dust particles frequently collided, losing some energy to heat.

同時，氣體和塵埃粒子經常發生碰撞，損失了一些能量，變成了熱量。

Because they slowed down, gravity was able to pull them closer to the galactic center.

由於它們的速度減慢，引力能夠把它們拉近銀河系中心。

So why did the Milky Way, like most galaxies, become flat and not spherical like stars and planets?

那麼，為什麼銀河系和大多數星系一樣，變得扁平，而不像恆星和行星那樣呈球形呢？

The answer lies in its angular momentum.

答案就在於它的角動量。

The laws of physics dictate that in the absence of external forces, the total angular momentum of an object or system has to stay the same over time.

物理定律規定，在沒有外力作用的情況下，物體或系統的總角動量必須在一段時間內保持不變。

In a star or planet, the spinning material is so dense that the outward pressure partners with gravity to create a mostly spherical shape.

在恆星或行星中，旋轉的物質非常緻密，向外的壓力與重力共同作用，形成了一個大體呈球形的形狀。

But emerging galaxies tend to have low densities and high angular momentum, meaning that the spinning motion is a more significant factor than the outward pressure.

但新出現的星系往往具有低密度和高角動量，這意味著旋轉運動是比外壓更重要的因素。

Indeed, if a galaxy has a lower angular momentum, an egg shape can form.

事實上，如果星系的角動量較小，就會形成蛋形。

In galaxies like the Milky Way, particles, instead of falling directly towards the center, tended to fall parallel to the axis of rotation as to keep the larger total angular momentum constant.

在像銀河系這樣的星系中，粒子不是直接落向中心，而是傾向於平行於旋轉軸下落，以保持較大的總角動量不變。

Over billions of years, the cloud of particles gradually fell, sped up in their orbits, and created a spinning disk.

經過數十億年的時間，粒子云逐漸下降，在其軌道上加速，形成了一個旋轉的圓盤。

Now what about the arms?

那麼手臂呢？

Spiral arms like the one Earth is in are regions where stars and gas are packed more tightly together.

像地球這樣的旋臂是恆星和氣體更緊密地聚集在一起的區域。

But the arms aren't static structures.

但手臂並不是一成不變的結構。

They're caused by zones of compression that travel through the galaxy as waves.

它們是由壓縮區造成的，壓縮區以波浪的形式穿過星系。

Just as a whirlpool has multiple peaks and troughs, a density wave in a galaxy has multiple regions of high density— the bright spirals— separated by regions of lower density.

就像漩渦有多個波峰和波谷一樣，星系中的密度波也有多個高密度區域--明亮的螺旋線--被低密度區域分隔開來。

And as the wave travels, different stars are constantly entering and leaving the spirals.

在波的傳播過程中，不斷有不同的恆星進入和離開螺旋。

Density waves can form in several ways.

密度波可以通過幾種方式形成。

For some galaxies, a nearby companion galaxy stirs the pot.

對於某些星系來說，附近的伴星系會攪動這個鍋。

Its gravitational pull breaks the disk's symmetry, generating a wave that could last for a billion years.

它的引力打破了圓盤的對稱性，產生的波可能會持續十億年。

For other galaxies, the presence of a small clump of tightly packed stars and gas within the galaxy can have a ripple effect, spontaneously giving rise to a wave.

對於其他星系來說，星系內存在一小團緊密結合的恆星和氣體會產生漣漪效應，自發形成波浪。

This is thought to be the cause of the Milky Way's spirals.

這被認為是銀河螺旋的成因。

In both scenarios, the galaxy's overall rotation bends the dense region into spirals, which rotate around the galaxy's center.

在這兩種情況下，星系的整體旋轉都會把緻密區域彎曲成螺旋狀，並圍繞星系中心旋轉。

Our solar system is orbiting the galactic center faster than the spiral arms.

我們的太陽系繞銀河系中心運行的速度快於旋臂。

We'll be moving deeper into our current arm for millions of years before eventually putting it in our rearview mirror.

在數百萬年的時間裡，我們將不斷深入我們目前的手臂，最終將其置於我們的後視鏡中。

And recent observations have added another wrinkle to the picture.

而最近的觀察結果又給這幅圖畫增添了新的色彩。

Rather than just one density wave, a typical spiral galaxy likely has two or more waves that overlap with each other and travel at different speeds.

一個典型的螺旋星系可能有兩個或更多的密度波，它們相互重疊，以不同的速度傳播，而不是隻有一個密度波。

The result would be spiral arms that last for tens or hundreds of millions of years before breaking apart and reforming.

其結果將是螺旋臂持續數千萬年或數億年，然後斷裂並重新形成。

This may be happening in the Milky Way, meaning that when the Earth formed 4.5 billion years ago, the spirals themselves may have looked entirely different than they do today.

這可能發生在銀河系中，也就是說，當地球在 45 億年前形成時，螺旋本身的樣子可能與現在完全不同。

In any case, our spirals won't last forever.

無論如何，我們的螺旋不會永遠持續下去。

About 5 billion years from now, the Milky Way will start to merge with the Andromeda Galaxy, throwing off the balance of angular momentum and creating an egg shape— the birth of a new era in our galaxy's history.

大約 50 億年後，銀河系將開始與仙女座星系合併，打破角動量的平衡，形成一個蛋形--銀河系歷史上一個新紀元的誕生。

And watch as the battle between two forces shapes the structure of the entire universe with this video.

通過這段視頻，您將看到兩種力量之間的較量塑造了整個宇宙的結構。